Tyrosine protein kinases are a family of enzymes involved in the regulation of cell growth and differentiation. The aberrant expression or regulation of tyrosine protein kinases has a demonstrated role in causing cell transformation. The study of these enzymes can lead to a better understanding of mechanisms involved in the control of normal cell growth as well as the biochemical events that result in neoplasia. The long-term objective of this proposal is to understand the biological function of a tyrosine protein kinase of molecular weight of 56,000 (pp56) that has been found in elevated levels in the lymphoma cell line, LSTRA. In pursuit of this goal, it is proposed to undertake a study of the properties of this enzyme. The interaction of pp56 with specific subcellular structures such as the plasma membrane or the cytoskeleton has important implications with regard to the possible intracellular substrates for pp56 and the manner in which the activity of pp56 might be regulated by extracellular agents. Thus the location of pp56 within LSTRA cells will be examined through the use of subcellular fractionation and immunofluorescence techniques. The distribution of pp56 among various cell types will be examined to determine if pp56 is unique to lymphoid cells of the T lymphocyte lineage or is more widely distributed among normal cells. The pp56 tyrosine protein kinase will be purified from LSTRA cells for studies of its molecular and enzymatic properties. A purified preparation of pp56 is also needed in order to study its ability to phosphorylate proteins that may be in vivo targets for tyrosine protein kinases. The properties of the in vitro reaction that results in the phosphorylation of pp56 on a tyrosine will be investigated to determine if this reaction is intrinsic to pp56 or is catalyzed by a second tyrosine protein kinase. The in vivo phosphorylation of pp56 will be examined to better understand the significance of the observed in vitro phosphorylation and to explore the possibility that pp56 is a substrate for other protein kinases. The results of the research of this grant will yield important information about the properties of pp56 that can provide a basis for understanding the role of pp56 in regulating cell growth. (B)